Root element is missing and XDocument and SeeOrigin.Begin

by jmorris 6. May 2009 02:45

I was writing some unit tests for a project I am currently working and kept getting the following non-useful exception:

I have seen the error before many times in the past, but I could not put my finger on what was causing it. The code is pretty simple, it just creates and instance of an object that implements IXmlSerializable passing in a stream which internally adds additional chicled object to the stream and then loads the result into a XmlReader. The the XmlReader then outputs the results to the standard console (note that a little later I add assertations to validate that the data was 'extracted' correctly):

 

Needless to say, this code failed every time on XDocument.Load with the 'Root element missing' exception.  I vaguely remember getting this error sometime in the past, but couldn't put my finger on it. I finally ran across this blog post and a light bulb went off.



After writer.Flush() is called the position of the stream is EOF. Before you can read the stream you must set the position of the stream to the begining. Note that calling writer.BaseStream.Position = 0 does _not_ work. You must call writer.BaseStream.Seek(0, SeekOrigin.Begin) to reset the streams position before creating the reader and loading the XDocument object. Also, note that the error only occurs because the XDocument object thknks the XML stream is invalid.

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Rendering Views using ASP.NET Webforms and Model View Presenter (MVP) Pattern and AJAX

by jmorris 1. May 2009 21:49

ASP.NET MVC introduces the notion of partial views, which either allow specific Actions to be called on a Controller and the resulting view outputted as HTML or allow Model Data to be passed from the current view to another, child view.

Calling Actions on a Controller and returning “sub” Views is incredible useful in situations were Ajax type behavior is required, since the resulting output is simply raw HTML. The HTML can then be appended to a DOM element via JQuery, some other JavaScript library or simply with straight JavaScript.

Unfortunately if you are still working on Web Forms projects or you have no need or desire to drink the kool-aid of MVC, you are pretty much out of luck for similar functionality built into Web Forms. However, it is fairly easy to implement this sort of behavior within the context of a Web Forms environment and then use a Ajax to render pure HTML from these “partial views.”

The code for doing this is surprising simple and was adapted from a Scott Guthrie posts a few years back:


In this case I allow a Dictionary containing the name value pairs of the properties on the View that will be loaded to be passed in as well. The key must match the name of a Property on the View and if it does the Property is set with the value of the key. This is useful in cases were you would like a parent View to pass data to a child View for initialization situations, etc.

I also have a static collection of Views defined which map a View’s name to a path in my applications root directory. RenderView looks up its path and loads the control before setting the Views data. In my case I load this from a table via a Data Access Object (DAO) in the Global.asax on Application_Start:



The PageUserControlDAO loads the View definitions from the database once when the application is started or the App Pool is restarted/refreshed.

Rendering the View involves calling the RenderView method and passing in the appropriate View name and a Dictionary containing the data to into the view:



In this case I used Page_Load to render the View into a panel (div) which isn’t too exciting. The really useful aspect however is when you combine JQuery and Web Services to render the View on demand from the client.

Rendering Views from the Client Using JQuery and Web Services

The Web Service:



The JQuery code to load the view and attach it to the DOM:


In the JavaScript above, renderView takes three parameters: the name of the View to render, the data to pass to the View, and the DOM container to load the view into. The $.ajax(..) method is a JQuery method that specifies the WebService to use, the method to call (RenderView) and the content type to specify in the request header.

When the method is called like so from the client:

 

renderView('ImageView', { 'CurrentPageElementId': 100 }, $view);


The WebService method is invoked, which renders the view and returns pure HTML that is then appended to the DOM element $view (a simple div or span).

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My first rant: R.I.P. Max 'The Dog' Sept 1995 - Oct 11 2008

by jmorris 12. October 2008 01:16

Got the call from my father today, that our family dog was not eating, unresponsive and breathing heavy. Bad signs. I drove to his house and poor Max was looking real bad. We drove him to the vet and they showed me several lumps on his stomache and side - I gave him a bath last month and didn't notice anything out of the ordinary. She said his temperature was over 107 (102 is expected, 103 is a fever in dog world).

Anyways, Max was a wonderful loving, protector of our family (he was family) for 13 years and we will miss him. Miss you buddy.

 



Max Sept. 1995-Oct 11 2008

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Jeff Morris

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